Low remanence high aluminum-iron



United States Patent LOW REMANENCE HIGH ALUMINUlVl-IRON Dusau Pavlovic, Forest Hills, and Karl Foster, Pittsburgh, Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Filed Nov. 21, 1958, Ser. No. 775,364

4 Claims. (Cl. 148121) This invention relates to aluminum-iron alloys to be used for electrical applications and in particular it concerns the preparation of high aluminum-iron alloys characterized by outstanding magnetic properties.

In our copending application Serial No. 678,517, filed August 16, 1957, we have disclosed low remanence, medium aluminum-iron alloys together with a method of producing them. In our copending application Serial No. 678,539, filed August 16, 1957, we have disclosed high aluminum-iron products and methods of producing them, characterized by outstanding magnetic properties including low remanence. It may be noted, upon consulting application Serial No. 678,539, that the method there disclosed is applicable to but a portion of the alloy compositions to which that application refers, where low remanence is an intended property. It may further be noted that the manner of characterizing the alloys with low remanence in each of the foregoing applications differs in each instance. lndeed in other studies that we have conducted, we have found that the procedures disclosed in those applications cannot be applied with similar success to alloys of a composition other than those identified therewith.

It is therefore apparent that it is not possible to pro duce, at will, magnetic aluminum-iron materials with exceptionally low remanence values without regard to the specific composition or processing steps used. ilndeed, a review of the art including our own prior applications shows that there are many aluminum-iron compositions for which no method at all is available.

It is therefore a major object of the present invention to provide aluminum-iron magnetic materials having an aluminum content of at least 16.5 percent and characterized by having, in addition to good magnetic properties in general, a very low remanence and a low coercive force.

This and other objects are attained in accordance with our discoveries relating to the application of a specific heat treatment to aluminum-iron alloys containing at least 16.5 percent of aluminum. In practicing this in vention, the alloy, in the form of a strip or sheet about 2 to 15 mils in thickness, is annealed in a vacuum at a high temperature. ilhereupon the annealed product is cooled to an intermediate temperature, held at that intermediate temperature for a defined soaking period and then quenched in water to room temperature. The resulting material is found to be characterized by outstanding magnetic properties particularly a low coercive force and including a low remanence, and as a consequence of these properties, is of particular utility for such applications as core material for current transformers, pulse transformers, meters and sensitive relays. For example, advantage of these properties can be utilized effectively in meters, and similar applications, which require a narrow hysteresis loop with constant permeability over a fairly large induction range.

The invention is practiced using a sheet or strip of the alloy having a thickness on the order of about 2 to 15 7 2,980,563 Patented Apr. 18, 1961 ice mils. Such asheet and strip can be obtained by hot rolling an ingot or plate of the alloy at a temperature above the recrystallization temperature, eg, at a temperature within the range of 800 to 1100 C. Other rolling schedules can-be used if desired.

The materials used in practicing this invention are aluminum-iron alloys containing at least 16.5 percent of aluminum and suitably aluminum within the range of 16.5 to 17.5 percent by weight. For many purposes, alloys containing 16.7 to 17.0 percent by weight of aluminum and the remainder iron are preferred. Other alloying constituents and incidental impurities may be present in varying amounts providing they do not deleteriously interfere with obtaining the described properties in the resulting products. A satisfactory method that we have used to prepare such alloys involves melting a relative pure iron, such as electrolytic flake iron, and then adding commercially pure aluminum. Normallythe iron is vacuum melted with the aluminum being added under conditions to minimize its loss, as by adding it under a blanket of helium. .The resulting ingot or plate is then rolled as stated above to provide the desired strip or sheet.

The high temperature phase of the treating schedule used in this invention is carried out in a vacuum 0n the order of 100 microns pressure or less. The material is vacuum and the other in dry hydrogen.

heated in this vacuum at a temperature within the range of about 1115 to 1300 C. for at least one hour and suitably for 1 /2 to 4 hours. In general it can be stated that at the higher temperatures shorter periods of heating are needed to obtain the described properties, while the time of heating would be lengthened at the lower temperatures. Thereupon the material is permitted to cool slowly to a temperature within the range of about 550 to 650 C. Preferably, this is accomplished in the furnace. The rate of cooling is not to exceed about 200 C. per hour.

When the intermediate temperature has been attained, the temperature is held at that point for about 5 to 30 minutes to insure equilibrium. Then the furnace is filled with an inert gas such as argon or helium. The productsthen are immediately quenched in water to room temperature.

The invention will be described further in conjunction with the following example in which the details are given by way of illustration and not by limitation.

EXAMPLE I An aluminum-iron alloy was prepared as follows: Electrolytic flake iron was melted in a magnesium oxide crucible in an induction heated, vacuum furnace that had been pumped down to a pressure of 0.10 micron. When the iron was thoroughly melted, commercially pure (99.99%) aluminum was added in a mount of 16.8 parts by weight for each 83.2 parts of iron. One-half inch thick plates were cast from this melt. The resulting plates were hot rolled at 1000 C. to sheet having a thickness of 0.007 inch. Standard ring laminations were punched from.- the sheet to provide two samples of stacked laminations. One sample was heat treated in a The following heat treatment schedule was used for each sample:

( 1) Heat at 1200 C. for two hours.

(2) Furnace cool to 600 C.

(3) Soak at quenching temperature for about 15 minutes.

(4) Fill furnace with argon or helium after soaking then immediately quench samples in water to room temperature.

D.-C. magnetic propertieswere determined on these heat treated ring laminations and the data obtained are as follows:

1 Taken from a field of 100 oersteds.

These data show that the vacuum treating schedule results in a remarkably low remanence value as compared with the hydrogen treating schedule and demonstrate the criticality in the atmosphere used. These data also show the generally superb magnetic properties of products prepared in accordance with out invention. In other words these data show that the products of this invention are characterized by high maximum permeability and very low remanence in addition to a low coercive force.

Other experiments have demonstrated the reproducibility of these results. Using this procedure and the described alloy composition, we have been able consistently to produce strip and sheet having a remanence value of less than 500 gausses measured from a field of 100 oersteds, a maximum permeability, in gauss per oersted, of at least 25,000 and frequently higher than 38,000 and a coercive force of at least as low as 0.1 and usually below 0.05 gauss at a field of 100 oersteds.

These unique products are seen to be of particular note upon comparing them with some of the finest commercial materials now in use. That has been done for 0.007 inch tape of this invention.

In order to compare the low-remanence, 16.8 percent aluminum-iron with oriented, 3 percent silicon iron for an application where a low remanence is required, a gapless core of low remanence, 16.8% aluminum iron should be compared with a core of oriented, 3 percent silicon iron containing a gap as required to provide a low remanence. As no data taken on a core or oriented, 3 percent silicon-iron containing an air gap were available, catalogue data of an uncut core of this material were used. It should be noted, however, that introducing an air gap in such a core will considerably degrade its magnetic properties over the values measured in a gapless core.

Exciting current and coreloss are the properties used conventionally in evaluating core materials for A.-C. applications; a low value of both properties is desirable. The AC. excitation required to bring material of our invention to an induction of 1 kilogauss is compared with oriented, 3 percent silicon-iron tape of three difierent thicknesses in Table II.

Table II A.C. EXOIIATION [R.M.S. volt-Amperes per pound at an induction of 1 kiloganss] The 0.007 inch thick, low-remanence, 16.8 percent. aluminum-iron ring laminations of this invention display a considerably lower A.-C. excitation at low inductions and at frequencies tested (up to kc.) than either 0.005 or 0.012 inch thick sheets of oriented, 3 percent silicon iron. A comparison of the subject material with 0.002 inch thick, oriented, 3 percent silicon-iron tape shows that 0.007 inch thick rings of low-remanence, 16.8 percent aluminum iron'require approximately 50 percent less excitation than 0.002 inch thick, oriented, 3 percent silicon-iron tape at frequencies up to 2.5 kc. At 5 kc., these two materials are identical but at 10 kc., the 0.002 inch thick, oriented, 3 percent silicon iron is slightly better. Thus, the 0.007 inch thick, low-remanence, 16.8 percent aluminum iron is better at lower,

frequencies and comparable at higher frequencies with 0.002 inch thick, oriented, 3 percent silicon-iron tape in A.-C. excitation in spite of the fact that the latter material is (a) 3 /2 times thinner than the ring laminations of the low-remanence, 16.8 percent aluminum iron, and (b) the cores of this material contained no air gap, a device conventionally used to obtain a. low remanence. Core loss data on our 7 mil material have 'been determined at frequencies up to 2 kc. and inductions up to 5 kilogausses. These are presented in Table HI. Core loss catalogue data for oriented 3 percent silicon-iron, 2 mil thick, and Tran-Cor 3 percent silicon-iron, 5 mil thick, are included in Table III for comparison.

(a) Low remanence 16.8% Al-Fe, 0.007 inch thick rings.

(0) Oriented 3% Si-Fe, 0.002 inch thick tape.

(c) Tran-cor 3% Si-Fe, 0.005 inch thick tape.

At all levels tested, 0.007 inch thick rings of lowremanence, 16.8 percent aluminum-iron of this invention display considerably lower core losses than 0.005 inch thick, 3 percent silicon-iron tape. At inductions up to 3 kilogausses, core losses of 16.8 percent aluminum-iron, as compared with those of the oriented, 3 percent siliconiron tape, 0.002 inch thick are identical at 400 c.p.s.,

comparable at 1000 c.p.s., and somewhat inferior at' 2000 c.p.s.

Another important factor .to be considered in the evaluation of these materials is the difference in density between 16 percent aluminum-iron and 3 percent siliconiron. A given volume of 16 percent aluminum-iron is 17 percent lighter than the same volume of 3 percent siliconiron. Thus, if 3 percent silicon-iron and 16.8 percent aluminum-iron are compared on a volume basis, instead of per unit weight as is customary, 16.8 percent aluminumiron would appear better because of its lower density.

The foregoing discussion and data clearly demonstrate the outstanding results achieved in accordance with our discoveries. The materials produced are capable of use in applications where oriented silicon-irons are now used.

In accordance with the provisions of the patent statutes, the principle of the invention has been described and there has been disclosed what is now believed to represent its best embodiment. However, it should be understood that the invention may be practiced otherwise than as specifically disclosed.

We claim as our invention: i

1. A method for preparing an aluminum-iron alloy having outstanding magnetic properties of high maximum permeability, low coercive force and a low remanence of below 500 gausses, when measured at oersteds,

assumes which comprises heating a magnetic alloy member consisting essentially of 16.5 to 17.5 percent by weight of aluminum and the remainder iron in a vacuum at a temperature of about 1115 to 1300 C. for at least one-half hour, slowly cooling the annealed alloy member to a temperature of about 550 to 650 C., releasing the vacuum on said alloy member and quenching the alloy to about room temperature.

2. A method in accordance with claim 1 in which said alloy that is treated has a thickness on the order of about 2 to 15 mils.

3. A method in accordance with claim 1 in which said aluminum is present in an amount of 16.7 to 17.0 percent.

4. An alloy strip characterized by a high maximum permeasured from a hysteresis loop taken at a field of 100 oersteds, and a coercive force of below about 0.1 oersted at a field of 100 oersteds, which alloy consists essentially of 16.5 to 17.5 weight percent of aluminum and the remainder iron, said alloy strip being of about 2 to 15 mils in thickness, said alloy strip having been produced by the process of claim 1.

References Cited in the file of this patent Navord Report 4130, The Fabrication and Properties of Alfenal Alloys (Al-Fe) Containing 10 to 17 Percent Aluminum, US. Naval Ordnance Laboratory, 5, December 1955, pages 10 and 7 and 13.

1948 Metals Handbook by the American Society for meability, a remanence value of below 500 gausses as 15 Metal, page 1161. j 

1. A METHOD FOR PREPARING AN ALUMINUM-IRON ALLOY HAVING OUTSTANDING MAGNETIC PROPERTIES OF HIGH MAXIMUM PERMEABILITY, LOW COERCIVE FORCE AND A LOW REMANENCE OF BELOW 500 GAUSSES, WHEN MEASURED AT 100 OERSTEDS, WHICH COMPRISES HEATING A MAGNETIC ALLOY MEMBER CONSISTING ESSENTIALLY OF 16.5 TO 17.5 PERCENT BY WEIGHT OF ALUMINUM AND THE REMAINDER IRON IN A VACUUM AT A TEMPERATURE OF ABOUT 1115* TO 1300*C. FOR AT LEAST ONE-HALF HOUR, SLOWLY COOLING THE ANNEALED ALLOY MEMBER TO A TEMPERATURE OF ABOUT 550* TO 650*C., RELEASING THE VACUUM ON SAID ALLOY MEMBER AND QUENCHING THE ALLOY TO ABOUT ROOM TEMPERATURE. 